1. Field of the Invention
The present invention relates to a feed-forward amplifier, and particularly relates to a feed-forward signal cancellation control loop therein, as well as a feed-forward loop control utilizing IF signal processing.
2. Description of the Related Art
In a classical feed-forward control system there are two independent control functions occurring simultaneously. The first is a signal cancellation loop and the other is a distortion cancellation loop. Generally two methods of loop control exist. The first method is signal adaptive and the second is control indirectly by use of a pilot or internally generated signal. Each approach has advantages and disadvantages. Adaptive approaches operate on the desired signals or the spurious byproducts of the signal. Pilot systems inject an internally generated signal at strategic nodes in the system, which when detected and nulled or reduced to a low value by control circuitry, balance the gain and phase response of the active path (amplifier) with the passive path (delay structure). This is called “feed-forward cancellation” and the nulling action optimizes the distortion canceling effect of the feed-forward amplifier system.
In most current feed-forward products, control of the first loop i.e., the signal cancellation loop, is adaptive. There are early designs that utilize pilot control, but they suffer from poor cancellation and pilot leakage out of the amplifier. The reasons for poor cancellation and leakage follow.
A typical pilot application is to inject a pilot signal at the input of an amplifier. In a feed-forward signal cancellation loop, the signal is split into two paths, one active (amplified) and one passive (delayed). The active signal is sampled and recombined anti-phase. The pilot, since it exists in both paths with equal amplitude, will be canceled or nulled when both paths are equal. Theoretically this meets the requirements for loop optimization or control.
Practical limitations are that since the pilot is passed through a non-perfect amplifier with non-linearities, the pilot and desired signal are inter-modulated. This inter-modulation prevents a satisfactory loop balance. A design goal of the signal cancellation loop is to minimize the power at the signal cancellation node. Presence of inter-modulation on the pilot prevents the actual pilot null from reaching the best signal cancellation. Another major drawback is the fact that since the signal is applied to the input of the system, the pilot is amplified along with the desired signals and appears at the system output, requiring a narrow band filter to remove this spurious pilot signal. This causes a loss of output power due to the finite Q of a practical filter structure.
In view of these drawbacks, the adaptive method has become the most widely used approach to control the signal cancellation loop. The adaptive method may employ a power detector to detect the average level at the signal cancellation node. A micro-controller adjusts the phase and amplitude or complex gain device until the power at the signal cancellation node is minimized. This is somewhat of an iterative process since the sampled signal has only amplitude information and no phase information. The controller must generate a complex (i.e., real and imaginary parts) control signal whereas only amplitude information is available. The controller must adjust phase and amplitude by trial and error to produce a signal null. Another issue is the fact that since the actual signal is sampled, it is of utmost importance that the sampling period be longer than any envelope variations in the signal. If the sampling period is close to the period of the variations in the envelope, the nulling algorithm will not be able to determine if a lower signal level is the result of a controller change or signal envelope change, causing an endless hunt to find a null point or possibly a loop oscillation. A remedy is to slow the sampling time down to one-tenth the slowest rate of change in the envelope. However, this limitation can adversely affect the transient response to fast signal level changes such as the rapid changes in the number of carriers present or the carrier power.
Another popular method of implementing the signal cancellation loop control is Cartesian loop or phase lock loop method, which offers a substantial increase in response time since it senses both phase and amplitude information simultaneously. The outputs to the complex amplitude controllers are generated directly and require no additional signal processing. The result is a 10:1 to 100:1 improvement in loop speed. Other advantages include insensitivity to signal level envelope variations. Despite these advantages of the Cartesian loop or phase lock loop method, there are limitations and drawbacks.
One limitation is that implementation of such a Cartesian loop is usually accomplished using signal diodes as phase detectors. It is well known that diodes have a definite threshold voltage and below that voltage, there is no output for a change in input. When diodes such as described are used in a signal cancellation loop, there is a definite signal threshold that must be reached before the loop can cancel the signal or balance the loop. This limits the dynamic range at low operating levels. Possible solutions employ the use of limiting amplifiers for at least the reference input to the loop. Although resulting in improved performance, this approach results in production of distortion products at relatively high levels due to the relatively high drive levels required to operate the diodes. These products can leak into high gain portions of the amplifier system and will not be removed by the distortion cancellation process.
A widely used method for distortion cancellation loop control in feed-forward amplifiers is the application of a pilot signal injected somewhere in the main signal path. The principle is to detect the pilot signal at the system output and by using control circuitry, null or reduce the pilot level in the system's output substantially. The pilot is nulled when the distortion cancellation path (error amp) is balanced with the main amplifier delay path. If phase and amplitude response is flat across the correction bandwidth, distortion components will be reduced at an optimum amount when the pilot, considered a distortion signal, is nulled. This method of indirect loop alignment has the advantage that it is independent of the amplified signals and the loop remains closed even during periods when no RF signals are present.
One of the earliest methods of loop control was to inject a continuous wave (“CW”) pilot at the carrier into the main path. A pilot receiver (which consists of a down converter, band pass filtered IF amplifier and detector,) detected the pilot. The detected pilot was used as an indication of loop balance or null. Simple hardware algorithms adjusted the phase and gain of the error loop by incrementing or decrementing the gain and phase adjustments until the pilot level was reduced to a low level. This method, relying on trial and error, results in prolonged loop lock time. Later techniques were developed which separated the pilot signal into quadrature components that could be used directly to control the gain and phase. By separating the signal into quadrature components, the direction and amplitude of the control signals are extracted directly without computation or iteration so the loop lockup time is reduced considerably. At least one technique was developed which applied two independent modulating signals on the pilot in quadrature. The pilot receiver output contained the two signals that represented phase and amplitude information. Frequency selective synchronous detectors recovered the phase and amplitude information to control the loop. The later techniques require calibrated cables or phase shifters that operate at RF frequencies.